1. Field of the Invention
This invention relates to a positive-type photosensitive electrodeposition coating composition, and more specifically, to an anionic or cationic electrodeposition coating composition suitable for forming a printed wiring photoresist by coating an electrically conductive material-clad laminated plate by electrodeposition to form a non-tacky smooth film whose portion exposed to actinic rays such as ultraviolet light through a positive photomask can be washed away with a developing solution.
2. Description of Related Art
In the prior art, a printed wiring board for use in an integrated circuit or the like is formed by plating a laminated plate obtained by cladding an insulator with an electrically conductive material such as a copper foil, laminating a photosensitive film, overlaying a photographic negative on it, exposing the photosensitive film through the negative, removing the unexposed portion, etching away the unnecessary electrically conductive material from the circuit pattern, and thereafter removing the photosensitive film on the circuit pattern. Since the photosensitive film is generally as thick as 50 micrometers, the circuit pattern formed by exposure and development is not sharp, and moreover, it is difficult to laminate the photosensitive film uniformly to the surface of the electrically conductive material. In particular, it is more difficult to protect the metal on through-hole portions with the photosensitive film.
A method is also known to form a circuit pattern for printed wiring which comprises applying an etching resist ink to a metal-clad laminated plate having a through-hole portion by screen printing, etching the laminated plate to remove metal from the non-printed portion, and removing the resist ink in the printed portion. According to this method, it is hard to form a pattern having a thickness of less than 200 micrometers with high reliability, the ink is difficult to coat on the through-hole portion, and the metal in the through-hole portion is frequently removed by the etching treatment. To avoid this, it is also the practice to embed an organic material in the through-hole portion so as to prevent the metal in the through-hole portion from being removed by the etching treatment, and finally remove the organic material. This method, however, has the defect that the cost of the circuit plate finally obtained is high and the circuit pattern has low sharpness.
As improvements over these prior methods, U.S. Pat. Nos. 4,632,900 and 4,673,458 disclose a method which comprises forming a positive-type photosensitive resin resist on a printed wiring board by electrodeposition, overlaying a photographic positive on it, exposing the board through the positive, and removing the exposed portion with an aqueous alkaline solution to form an image. Since according to this method, a coating can be formed easily on the through-hole portion by electrodeposition and the unexposed portion remains as a resist coating, a printed wiring board having excellent resolution can be obtained.
In the above-cited U.S. Pat. No. 4,632,900, polyoxymethylene polymer, o-nitrocarbinol ester, o-nitrophenyl acetal, and a quinonediazidesulfonyl ester of novolak resin are used as the resin in the resin composition for forming the photosensitive resin resist by electrodeposition. In U.S. Pat. No. 4,673,458, a resin obtained by esterifying a hydroxyl group contained in an unsaturated monomer with a sulfonic acid group contained in naphthoquinone diazidesulfonic acid, and copolymerizing the resulting unsaturated monomer with another unsaturated monomer is used as the above resin. However, the former method cannot produce a high-density fine pattern circuit board of sufficient reliability whichever resin may be used. Moreover, since the electrodeposition paint has insufficient stability, flocculation is liable to occur and tends to cause filter clogging or imperfections on the coated surface upon long-term running of the electrodeposition bath. In the latter method, a photosensitive naphthoquinone diazide group is introduced into the resin through a sulfonic acid ester group. Hence, when the electrodeposition is carried out over a long period of time (the turnover of the electrodeposition paint is long), the sulfonic acid ester group in the resin is easily hydrolyzed by a hydrolytic substance in the electrodeposition bath such as water, an acid, a base or an alcohol to degrade the resin frequently. As a result, the resin component flocculates in the electrodeposition bath or precipitates at the bottom of the bath. This causes filter clogging or greatly varies the electrodeposition characteristics such as application voltage. Alternatively, abnormal electrodeposition such as pinholing occurs, and the electrodeposition coating bath becomes difficult to control. Moreover, the coated film formed from the electrodeposition bath has poor smoothness and alkali developability, and it is impossible to obtain a printed wiring board having excellent resolution.